1. Field of the Invention
This invention is in the field of removal of dissolved materials from water by the reverse osmosis technique implemented with an applied alternating electric field to improve the flow rate through the membrane.
2. Description of the Prior Art
The reverse osmosis process for water desalination has become of growing importance. The main reason for this importance is that in contrast to the normal methods for water desalination, the energy consumed by the reverse osmosis process is very small. However, a disadvantage of the process is that the flow per day and area of the active surface of the membrane is rather small. In the past twenty years, the development of the reverse osmosis process has proceeded in two principal directions. In one, small flow rate across the membrane is compensated for by increasing the membrane surface per unit of volume in a desalination module. This approach has been quite successful in the use of a hollow fiber system which uses billions of fibers in one module, producing a large osmotic surface so that a small flow rate is acceptable.
Another way for improving the through-put is to increase the water flow across the membrane. For this purpose, there was developed a multitude of synthetic polymer resins as the basic membrane materials. In recent times, this synthetic polymer material was pigmented with minerals, metal oxides or metal hydroxides, using the interfacial effects of the pigment-membrane-resin interaction.
It is well known that after a short working life, the flow across the membrane decreases not only due to the compaction effect on the membrane by high pressure but also by an increase of Donan potential. The Donan potential is the result of accumulation of salt or salt solutions across the membrane, particularly on the feed side surface of the membrane. These concentrations produce a potential which acts in opposition to the desalination reaction.
The accumulation of salt along the membrane surface must be eliminated without damaging the product. Consequently, there have been many suggestions in the art on how to eliminate superficial salt accumulations. Some of the means suggested have been using a high velocity of the feed fluid alongside the membrane surface, oscillation of the membrane, or using ultrasonic waves directed toward the membrane surface.
All of these means produce a certain improvement but they cannot remove the ultimate salt layers from the surface and the interior of the membrane. Consequently, a reduced yield may result.